Magnetic resonance (MR) imaging systems are often used for the examination of patients, further referred to in general as subject of interest. In terms of imaging, the subject of interest is also referred to as a load. By such a system, the nuclear spins of the body tissue to be examined are aligned by a static main magnetic field B0 and are excited by transverse magnetic fields B1 oscillating in the radio frequency band. In MR imaging, relaxation signals are exposed to gradient magnetic fields to localize the resultant resonance. The relaxation signals are received and reconstructed into a single or multi-dimensional image. Furthermore, in MR spectroscopy systems, information about the composition of the tissue, which is carried in the frequency component of the resonance signals, is further evaluated to obtain additional information.
Typically, a radio frequency (RF) coil system is provided for transmission of RF signals and for reception of resonance signals from the subject of interest. The RF coil system may comprise a single RF coil element, e.g. a body coil, which may be integral part of the MR imaging system. In addition or alternatively, the RF coil system may comprise special purpose coils, which can e.g. be flexibly arranged around the subject of interest or in a specific region of the subject of interest to be examined. Special purpose coils are designed to optimize signal-to-noise ratio (SNR), particularly in situations where homogeneous excitation and high sensitivity detection is required. Furthermore, special sequences of RF signals, increased field strengths, high flip angles or real-time sequences can be realized and generated using multi-element coil arrays, which comprise multiple independent RF coil elements, which provide the possibility for multi-channel transmission and/or reception of RF signals.
In state of the Art MR imaging systems, the use of RF coil systems with multi-element RF coil arrays is becoming more and more common. The use of such RF coil systems can improve B1 magnetic field homogeneity and reduce specific absorption rate (SAR) in subjects of interest, which permits operation at high field strengths, e.g. 3 Tesla (T) or even higher.
Today, the clinical application of multi channel transmission is RF-shimming at 3 T, since wave propagation effects generate big variations in many subjects of interest. RF shimming is needed to improve the homogeneity of the transmit field amplitude and enables clinical investigation with even with wave propagation effects present within the volume of interest of the subject of interest. Basic idea of RF-shimming is to superimpose various transmit fields with different shapes, phase and amplitude so that the sum of the individual transmit field amplitudes becomes homogeneous inside a desired FOV covering the subject of interest and additionally obtain a optimization for reduced total and local absorbed RF energy (SAR specific absorption rate) in the human subject.
Those transmit fields are typically generated by a multi-element RF coil array. One of the challenges in the development of such a RF coil arrays is to improve the power efficiency. Due to different position and loading, efficient individual RF power matching is required. A further clinical application is the use of individual transmit pulses for MR imaging (Transmit Sense). These pulses differ in envelope, phase, amplitude, frequency and time.
US-patent application US2013/0285659 (Sohn) discloses an RF coil assembly and discusses sampling of RF power from the RF signal path and employs the ratio of forward and reflected power to control the tuning and matching.